Preheat oven to 220c or gas mark 7.
In a large bowl, whisk together eggs and milk and season with salt and pepper.
In a 12-inch oven-safe, nonstick frying pan, melt 2 tablespoons butter over medium-high heat. Add bread and cook, turning, until golden on all sides, for about 5 minutes. Transfer to a plate.
Add the remaining tablespoon of butter to the frying pan and add the mushrooms, cook on a high heat until mushrooms are soft and the water has evaporated.
Lower the heat to medium, add spring onion whites, and cook until soft. Season.
Add the whisked eggs to the frying pan, stir gently, once the eggs have begun to set and congeal but are still soft add the bread and gently mix in. Leave some pieces of the bread above the egg mixture and other pieces completely submerged.
Sprinkle with cheddar cheese and bake in the oven for around 6/8 minutes or until eggs have set and mixture has risen slightly (puffed up).
Top with remaining spring onions and serve.